Gallery, Projects and General => The Design Shop => Topic started by: BillTodd on December 01, 2010, 03:19:18 PM

Title: LTD stirling concept
Post by: BillTodd on December 01, 2010, 03:19:18 PM
Too cold here to be out in the shed, so I thought I'd machine some pixellanium and run this idea past the assembled experts...

This is obviously only a concept drawing, but it includes a couple of ideas inspired by Picclocks excellent horizontal LTDS thread.

I've used a scotch yoke to drive the displacer to provide extra dwell during the heating/cooling parts of the cycle, and a simple straight line linkage to the displacer rod.

All comments welcome :)


[edit] Sorry picclock, you're a writer of code, not a picker of locks :)
Title: Re: LTD stirling concept
Post by: cidrontmg on December 01, 2010, 06:57:35 PM
I´m not sure it would much matter, but in principle, the displacer dwell times with this kind of linkage would not be symmetrical. The displacer would be snapped up very quickly, and then slowly lowered down. Gamma Stirlings are actually "double acting", in the sense that both the overpressure of the expanding heating air, as well as the under-pressure of the shrinking cooling air, are both utilized. Both drive the power piston to the desired direction. Although not by the same force. The suction is usually weaker, because of the lesser temperature difference. With the usual crank arrangement, both strokes happen in +/- equal time spans. Here that would not be. The displacer would be snapped high, and held high for longer than on the cooling side. I believe that would lead to a higher overall temperature and pressure in the engine - not necessarily a bad thing at all. Heating would happen for a lot longer than cooling. I really can´t say if this arrangement, where there´s more time to heat the air than to cool it, would be of benefit or detriment! In a slow running LTD Stirling, it might lead to a non-runner, though (an even air temperature in every part of the engine), unless there would be something cold (ice, evaporating liquid, cold water, etc.) above the displacer. But then it would no longer be an LTD... Nothing like experimenting, though!
Title: Re: LTD stirling concept
Post by: picclock on December 02, 2010, 11:06:02 AM
Hi Bill

I seem to have so many hats I need a bigger hatstand  ::).  However the thread was about a vertical shaft LTD.

Your drawing was impressive - which program did you use to create it on ?

Its something I ought to get to grips with. With 9 1/2" of snow I've no desire to make the trek to the shed at the end of the garden, so now may be a good opportunity.

Been thinking about  the economiser idea, fattening the displacer piston and filling the inside with aluminium mesh or whatever to make a regenerator. By doing this less heat /cold would need to be added/subtracted from the working fluid which would reduce the thermal impedance issue as less energy would need to be transferred. The outer sides of the displacer could be made from foam with a cutout in each side, diametrically opposite, to force the working fluid to pass through a long cavity with the mesh.

One part of the current designs for coffee cup engines I find fundamentally flawed is the use of a base made of aluminium. Only the part over the cup needs to be aluminium, the surrounding part could be a ring of wood/ plastic or any other reasonably insulating material. Just changing this will result in an increase of available energy and a longer running time. A base of wood with a green felt bottom which had a 90mm hole with a 95mm recess for the circular aluminium plate and displacer cylinder, would be a functional and aesthetic improvement. (see dodgy skectch). I'm not sure that I'm up to making that but if I can pick up a preshaped circular base putting a hole in it should be a piece of cake. Of course a wooden base would preclude the use of running it off of a tea light.

Best Regards


Title: Re: LTD stirling concept
Post by: madjackghengis on February 06, 2011, 08:28:06 PM
Hi all, just as a matter of general principle, your dwell time/surface area for both the heat and the cold surfaces fairly equal all factors considered.  I would think you'd be better off with equal factoring between dwell, temp differential, and surface area to keep the engine balanced fairly evenly.  As for regenerator material, stainless steel is the ticket as it absorbs it and transfers it very poorly, so it won't transfer heat from hot to cold side by conduction, and the movement of air through the material is relatively constant, just changing directions kind of like a sine wave if you graphed it over time.  The smoother the cycle, the less energy absorbed by the bearings and available for drive.  Just some thoughts I've had on the subject, not as if I'm an expert, just interested amateur. :smart: mad jack
Title: Re: LTD stirling concept
Post by: picclock on February 07, 2011, 06:27:51 AM
Hi Madjack

Just ordered parts to manufacture a prototype to evaluate the efficiency / energy gains using some of these techniques.

Finding it hard to get graphite though (Noggin End seem a bit pricey and SWMBO would go bonkers if I told her I paid £15 for 3" of graphite rod). My son is over in the states so I'm hoping he can get some from :

It'll still cost £12-13 but I'll get 4x as much and SWMBO may not find out  ::) .

Bit fed up at the moment as my mill motor decided to get very hot and then started tripping the mains breaker  :zap:. Waiting for new one under warranty so a bit stir crazy as I have a lot to do and no machine to do it on :scratch:


Acrylic tube and test tubes arrived as I posted. Best way to hold and cut 110mm acrylic tube without marking the surface anyone ?

Title: Re: LTD stirling concept
Post by: madjackghengis on February 16, 2011, 09:38:39 AM
Hi Picclock, I think I'd use masking tape around the acrylic tube, cutting it off on the band saw or hack saw, and then facing the ends in a three jaw. jack